Log feed rate is a critical parameter in the economics of sawmill operation. The large bandsaws used by the lumber industry reduce logs to useable lumber sizes. Operators manipulate the velocity of the logs through the saw, but deviations in the cut due to grain, knots, saw wear, wetness, and mechanical drive-related problems decrease the throughput and useable product by requiring a slowdown of the feed rate. Even with the best currently available equipment, an oscillation of the sawblade in the cut, called “snaking”, can develop due to excessive feed velocity.
Optimal performance will require feedspeed adjustments faster than an operator can respond, and currently available automated systems do not have feedback means to make timely adjustments in feedspeed. Typically feed velocities based on depth-of-cut are set conservatively to avoid overfeed. Only a few feed systems monitor power consumption of the saw motor for feedback. In these cases the sawblade must be entered into the cut conservatively and feed ramped up slowly while watching power consumption. Throughput must be conservative to avoid overfeed. Any speed adjustments due to “snaking” or “washboarding” can result in poorer tolerance control and production losses. Speed is strongly influenced by the conditions and overall dimensions of the log, which may vary from several feet in diameter at one end, to a fraction of that at the other. Some automatic feed systems compensate for this change in depth-of-cut by using a constant gullet fill curve to calculate a feed velocity profile along the log. The same log likely is denser at its base than at its crown end and the wood may be dryer at one end or the other, leading to blade deviation if overfed. The greater the diameter of the log at the base, the greater the depth-of-cut, and the slower the feed. Feedspeed may be increased as the blade cuts into the smaller diameter, less dense, opposite end of the log. Grain is also not uniform. The base of the log may have the grain of the wood and the sap rings angling out so that the sawblade cuts across alternating rings of varying densities of material, for example, and thus varies along the length of the log. Woods such as Douglas Fir are noted for twisted grain and knots. These factors can result in unequal forces on the swage tips, requiring slower feed velocity to avoid displacing the blade from a true cut, and resulting in uneven or angled product that must be corrected by reductive planing in order to have saleable lumber.
It would seem that automation would offer a means for speeding production throughput, but initial efforts to develop methods for controlling feedspeed by measuring bandmill power consumption failed because the inertial resistance to any change in RPM of the bandsaw wheels, which may be 6 ft in diameter, is too large for effective feedback control of drive power and lags the changes that occur (in milliseconds) during early deviation from a true cut. Thus power control loops have proven sluggish in response time and are unsatisfactory.
However, the advantages of controlling feedspeed have been recognized. A number of motion control systems have been proposed, including U.S. Pat. No. 4,926,917 to Kirbach, which discloses the use of two saw line light lines, one at the entrance end of a log and the other at the exit end on the opposite side of the log, to determine the thickness of the log. A microprocessor produces an electric signal for controlling the speed of feed of the log through the band saw. An electromagnetic proximity sensor is employed to measure lateral deviation of the sawblade as it enters the cut as an indication to an operator of dull saw teeth or other problem with the blade.
In U.S. Pat. No. 5,694,821 to Smith, the depth-of-cut measurement is similar to that disclosed in the patent discussed above, an electromagnetic proximity sensor detects lateral deviation of a band sawblade as it enters the cut, and the electric signal is processed to provide a corresponding electric drive motor signal to increase or decrease the speed of a log being moved through the band saw. Because the blade deflection is monitored at the entry to the cut in these systems, feedback control is limited and delayed.
U.S. Pat. No. 6,681,672 to Myrfield has met with commercial success, and is co-owned by the inventor. The feed control system uses a laser line to allow video scanning of the diameter profile of a log on the carriage so as to measure the impending depth-of-cut by the band sawblade. A second laser line is arranged light with a reference point on the band sawblade for detecting the lateral deviation of the blade from its true cut path. A video camera with image analysis software for mapping log coordinates and speed is arranged to cover the diameter of a log on the carriage and to view the laser line projections. Output from the imaging system is fed into an algorithm to control the speed of the power drive motor of the carriage. Optical means have also been commercialized by SiCam Systems (Delta, British Columbia, Calif.), for lumber quality control, offering an electromagnetic proximity sensor to detect lateral deviation at the upper sawguide. Because any blade deflection is monitored at the entry into the cut, any capacity to provide feedback control would be limited and delayed; likely resulting in snaking.
However, the bandsaw control systems of the above disclosures are designed to measure displacement of the sawblade proximate to the point where a tooth enters the cut. But any displacement of the saw blade at the top of the cut is preceded by a change in lateral force at the bottom of the cut and displacement of the blade inherently cannot occur until the tooth has sawn through the cut. Therefore, displacement measurements made above the cut are “trailing” or “lagging” measurements. Lagging signals are inadequate for closed loop control of velocity, particularly at higher feedspeed where incipient deviation occurs in microseconds. Thus, there is a need in the art for a bandsaw feed rate controller with a sawguide force sensor that overcomes the above disadvantages.